Gerrit. This information would be contained within the SMTP logging, and would look similar to this.
[2016.05.01] 12:00:00 [10.1.9.71][#######] Authenticating as rod@hostedsmartermail.com
[2016.05.01] 12:00:00 [10.1.9.71][#######] rsp: 334 UGFzc3dvcmQ6
[2016.05.01] 12:00:00 [10.1.9.71][#######] rsp: 235 Authentication successful
[2016.05.01] 12:00:00 [10.1.9.71][#######] Authenticated as rod@hostedsmartermail.com
However, this functionality wasn't added until SmarterMail 10. I would recommend upgrading your installation.